The invention relates to a method for generating polarized laser radiation in accordance with Patent claim 1, and to a laser resonator for generating a polarized laser radiation in accordance with the respective preamble of Patent claim 5 or 11, respectively.
Resonator mirrors are understood as mirrors between which a radiation field oscillates. Each oscillator generally has a highly reflecting mirror and an output mirror exhibiting a somewhat lower reflection via which a portion of the radiation field is output to be used for the most varied aims. Mirrors inside the resonator which are used to deflect radiation or for other purposes are not understood here as resonator mirrors.
In the method according to the invention described below, or in the laser resonator according to the invention, a polarization state or a polarization distribution are understood as linear and circular polarizations. Also included here, however, are radially extending or aligned polarization states and tangentially extending or aligned ones such as can occur, in particular, in the case of cylindrically symmetrically pumped, thermally birefringent laser rods. Also included here, moreover, are other arbitrary directions of polarization which vary with location over the beam cross section.
The object of the invention is to create a laser resonator of simple design for the purpose of generating polarized radiation which exhibits high efficiency and ease of adjustment and thus excellent radiation stability.
The object is achieved by virtue of the fact that of the radiation fields oscillating in the laser resonator, it is only one radiation field (radiation) exhibiting a prescribed polarization distribution that is partially output. The remaining radiations remain oscillating in the resonator with the inclusion of the proportion, remaining in the resonator, of the partially output radiation. Use is made for this purpose of a laser resonator exhibiting a highly reflecting resonator mirror and a radiation output device as well as an active medium. The radiation output device is designed in such a way that of the radiation fields oscillating in the resonator and exhibiting an arbitrary polarization it is only a prescribed, in particular a single polarization that can be output from the resonator with a prescribed output level. All differently polarized radiation fields as well as the respective remaining radiation field of the partially output radiation remain reflecting completely in the resonator up to a tolerance.
All the radiation fields in the laser resonator are preferably caused to oscillate between the two resonator mirrors.
Energy transfer is preferably undertaken between the radiation fields in the laser resonator. This energy transfer can be undertaken with a phase delay in a prescribed direction of polarization. Thus, the energy transfer element can be a birefringent element, preferably a thermally birefringent laser crystal, a xcex/4 plate or a Faraday rotator. The various uses are examined below.
It is also possible to use a nonlinear optical element in the resonator for outputting radiation. Frequency multiplication can then be undertaken with the aid of the element. The output mirror then preferably transmits completely only the frequency-multiplied radiation.
In the case of previously known laser resonators exhibiting thermally induced birefringent solid state laser media such as are preferably used in the case of high power lasers, great efforts have been made to compensate precisely this thermally induced birefringence. By contrast with known laser resonators exhibiting an active medium with a thermally induced birefringence, the invention now proposes a different approach. Specifically, in the case of the invention it is no longer necessary to compensate the thermal birefringence; by contrast, it is utilized. In some design variants, it is precisely the thermally induced, birefringent active solid state medium that is utilized, inter alia, as an element for energy transfer between the differently polarized radiation fields in the resonator. It is to be noted that in one design variant all the radiation fields are retroreflected into themselves in the reflector, and only the radiation field exhibiting the desired polarization state or a desired polarization distribution is output with a prescribed transmission.
Optical resonators in which the thermally induced birefringence of the active medium is compensated by optical elements in the resonator are illustrated and described, for example in N. Hodgson, H. Weber, xe2x80x9cOptical Resonatorsxe2x80x9d, starting with page 298, Springer-Verlag 1997, and in DE-A 44 15 511.
It would also be possible for a plurality of prescribed directions of polarization to be output in the case of the invention; however, it is preferred to restrict oneself to only one.
The radiation output device of the invention preferably exhibits an output mirror which transmits only a laser radiation exhibiting a prescribed polarization state or polarization distribution and a prescribed wavelength with a prescribed transmittance. Such a mirror is described, for example, in Rong-Chung Tyan, Pang-Chen Sun, Axel Scherer and Yeshayahu Fainman xe2x80x9cPolarizing Beam Splitter Based on the Anisotropic Spectral Reflectivity Characteristic of Form-birefringent Multilayer Gratingsxe2x80x9d, Optics Letters, Vol. 21, No. 10, May 15, 1996, pages 761 to 763, and in N. Bel""tyugov et al., SPIE Vol. 1782, 1992, 206-212. Such a mirror is also described in PCT/EP 00/07540.
It is also possible to make use as radiation output device of a nonlinear optical element which preferably performs in the resonator frequency multiplication of the radiation fields oscillating there. Use is then made as output mirror of a mirror which transmits, in particular completely, only one of the frequency-multiplied radiation fields having a prescribed transmission factor.
As already indicated above, an optical element exhibiting birefringence, in particular thermally induced birefringence, can be used for energy transfer between the radiation fields in the laser resonator. That is to say, in the case of solid state lasers it is possible, for example, for the thermally birefringent laser crystal already to serve as active medium for the energy transfer.
The energy transfer element can also exhibit a phase-delaying (phase-rotating action) for the radiation fields oscillating in the resonator, and can preferably be designed as a xcex/4 plate or exhibit the optical action thereof. It can also be designed as a Faraday rotator.
As the theory set forth below shows, a depolarization of greater than 30% per resonator round-trip pass of the resonator radiation fields should be reached in the laser resonator according to the invention. In general, the thermally induced birefringence generates strong coupling in the case of high-power laser crystals. In the case of low pumping powers, in which the depolarization is slight, it is possible to achieve a xe2x80x9csufficientxe2x80x9d depolarization by means of additional elements such as a xcex/4 plate, a Faraday rotator, etc.
However, there is also the possibility that of the radiation fields which can be built up in the resonator, it is only to be those which are not depolarized by the thermally induced birefringence of the active medium that are fed back up to an output level. In this case, at least one of the two resonator mirrors reflects only radiation fields having a polarization distribution which is not subjected to any depolarization on passage through the active medium, and does not reflect all radiation fields having other polarization states or also only to a slight extent which does not suffice to produce build up, such that no other radiation fields having different polarization states can buildup in the resonator. The highly reflecting mirror or the output mirror or (although this yields no advantage) both mirrors can now be provided with such a mirror coating.
High-power lasers exhibiting good beam quality can be produced with the aid of the laser resonator according to the invention or design variants thereof. A good beam quality, for example, with circular polarization, is used, for example, for material cutting in the case of industrial lasers.